Plating baths for the electroless deposition of copper are common and typically comprise a copper salt, e.g. copper sulfate, a complexing agent such as EDTA and a formaldehyde reducing agent. As copper is depleted from such plating baths, formaldehyde is oxidized to a formate and excess sulfate ions are generated. Copper ions and formaldehyde can be added to replenish a plating bath. Removal of formate and sulfate ions can be effected by withdrawing a purge stream. Besides the unwanted formate and sulfate ions, a purge stream also contains valuable species such as copper ions, formaldehyde and EDTA. Environmental awareness of the robustness of EDTA for the mobilization of toxic metals makes it less desirable to dispose of EDTA-containing solutions in waste streams.
Spent plating baths are traditionally treated by adding reducing agent such as sodium borohydride to precipitate elemental metal; residual soluble metal is precipitated with strong complexing agents, e.g. dithiocarbamate. Metal chelant complexes can be decomposed by oxidizing chelants, e.g. with peroxides, perchlorates or other oxidizing acids.
Because of the difficulties in treating spent plating baths, disposal in landfills is often a method of choice for disposing of spent plating solutions or metal sludge precipitate from plating baths. For instance, metal recyclers often prefer to avoid spent electroless nickel solutions because of the high phosphorus content.
U.S. Pat. No. 5,039,497 discloses methods of removing copper from sulfate solutions using aliphatic oximes. Cognis, Inc. (Santa Rosa, Calif.) has disclosed that such an extraction process can be used to treat copper and nickel electroless solutions to reduce the metal content producing a solution suitable for disposal, e.g. by sewering. Such solvent extraction methods have not been enthusiastically adopted for treating plating baths comprising copper complexed with EDTA, in part because common commercial extractants are not especially effective in extracting copper from complexes with EDTA.
Cardotte in U.S. Pat. No. 4,985,661 discloses the use of hyperfiltration membranes to process copper electroless plating solutions, e.g. to concentrate for re-use salts of EDTA. Such membranes are more permeable to formaldehyde and formate ions than EDTA salts. It has been found that a sufficiently high level of copper salts permeate such membranes both as formate salts and EDTA salts. Such copper-containing permeate streams are unsuitable for waste disposal in many places. Moreover, such EDTA-concentrated streams are typically unsuited for recycle without further treatment, e.g. to remove another anions, most commonly sulfate which is present as the principal copper counterion.